1. Field of the Invention
This invention relates to a reactive power compensation device for suppressing voltage variation in an AC power supply system caused by sharp variations of reactive power in the load.
2. Description of the Prior Art
When reactive power variations are irregular and large, as in an arc furnace, effective compensation of these variations necessitates rapid detection of the reactive power in the load and determination of the firing phase of thyristors of a reactive power compensation device in accordance with the detected reactive power so as to make the response of the reactive power compensation device to the variations as fast as possible.
A conventional reactive power compensation device is now discussed referring to FIG. 1. A load 10 of varying reactive power is connected to an AC power supply 12. The power supply 12 has a power source 14 and a system impedance 16 in series. A reactive power compensation circuit 18 is connected in parallel with the load 10. The compensation circuit 18 has a phase-lead capacitor 20 and a reactor 22 connected in parallel, and a pair of anti-parallel thyristors 24 connected in series with the reactor 22.
A voltage detector 26 is arranged to detect the voltage applied on the load 10, and a current transducer 28 is arranged to detect the current "i" flowing into the load 10.
The thyristors 24 are controlled by a controller 30 based on the output signals of the voltage detector 26 and the current transducer 28. Since lagging-phase reactive power Q.sub.L received by the reactor 22 from the power supply 12 can be varied by the thyristors 24 through arbitrary phase control of the firing angle .alpha. (alpha) in the range 0-100% as shown in FIG. 2, a reactive power compensation device is formed which compensates for the lagging-phase reactive power Q.sub.F of the load 10 through a combination with the leading-phase reactive power Q.sub.C in the capacitor 20.
For example, if the leading-phase reactive power of the phase-lead capacitor 20 is designated as Q.sub.C and the lagging-phase reactive power of the reactor 22 as Q.sub.L, the total leading-phase reactive power Q.sub.T received from the power supply is EQU Q.sub.T =Q.sub.C -Q.sub.L ( 1)
Q.sub.L is a function of the firing angle .alpha. of the antiparallel thyristors, as shown in FIG. 2. Therefore, it is obvious from Eq. (1) that Q.sub.T is also a function of .alpha., as shown in FIG. 3, and that Q.sub.T can be controlled by controlling the firing angle .alpha..
The overall reactive power Q.sub.S received from the power supply 12 is EQU Q.sub.S =Q.sub.F +Q.sub.T ( 2)
If the reactive power of the load 10, Q.sub.F, is detected and Q.sub.T is controlled to be equal to -Q.sub.F, Q.sub.S becomes zero, and the lagging-phase reactive power Q.sub.F is supplied from the reactive power compensation device.
Referring to FIG. 1, the controller 30 has a reactive power detector 32, a function generator 34, a comparator 36 and a pulse amplifier 38.
The reactive power detector 32 evaluates the reactive power Q.sub.F in the load 10 by the voltage measured by the voltage detector 26 and the current i measured by the current transducer 28.
The function generator 34 is a circuit which generates Q.sub.T of Eq. (1) as a function of the firing angle .alpha., as shown in FIG. 3.
The comparator 36 compares the two output signals of the reactive power detector 32 and of the function generator 34, and sends a pulse when the two output signals coincide. The pulse amplifier 38 amplifies the pulse to supply a gate signal to the thyristors 24.
The control is effected in a manner such that Q.sub.T compensates for the reactive power Q.sub.F of the load 10, as shown in FIG. 4. However, as seen in FIG. 4, with a thyristor phase-control reactor type reactive power compensation device in which reactive power consumed by a reactor is controlled by thyristors as described above, an opportunity for firing the thyristors occurs only twice in each power supply voltage cycle. Therefore, in principle, this control comprises half a cycle of dead time. In control of voltage flicker caused by reactive power variation of a load as typified by an arc furnace, this dead time is not negligible and so control effects are unsatisfactory.